Generally, various rearview mirrors for motor vehicles have been proposed which change from the full reflectance mode (day) to the partial reflectance mode(s) (night) for glare-protection purposes from light emanating from the headlights of vehicles approaching from the rear. The reflectance is varied by positioning a transmittance altering device between the viewer and the reflector. Among such devices are those wherein the transmittance is varied by thermochromic, photochromic, or electro-optic means (e.g., liquid crystal, dipolar suspension, electrophoretic, or electrochromic), and where the variable transmittance characteristic affects electromagnetic radiation that at least partly includes the visible spectrum. Devices of reversibly variable transmittance to electromagnetic radiation have been proposed as the variable transmittance element in variable transmittance light-filters, variable reflectance mirrors, and display devices, which employ such light-filters or mirrors in conveying information. These variable transmittance light filters have included windows. mirrors, and display devices, which employ such light-filters or mirrors in conveying information. These variable transmittance light filters have included windows.
In the past, information, images or symbols from displays, such as vacuum fluorescent displays, have been displayed in electrochromic rearview mirrors for motor vehicles with reflective layers on the third surface or fourth surface of the mirror. In one such device the display is visible to the vehicle occupant by removing all of the reflective layer on a portion of the selected layer and placing the display in that area. FIG. 1 shows a one type of electrochromic mirror device 10, having front and rear planar elements 12 and 16, respectively. A transparent conductive coating 14 is placed on the rear face of the front element 12, and another transparent conductive coating 18 is placed on the front face of rear element 16. A reflector (20a, 20b and 20c), typically having a silver metal layer 20a covered by a protective copper metal layer 20b, and one or more layers of protective paint 20c, is disposed on the rear face of the rear element 16. For clarity of description of such a structure, the front surface of the front glass element is sometimes referred to as the first surface, and the inside surface of the front glass element is sometimes referred to as the second surface. The inside surface of the rear glass element is sometimes referred to as the third surface, and the back surface of the rear glass element is sometimes referred to as the fourth surface. The front and rear elements are held in a parallel and spaced-apart relationship by seal 22, thereby creating a chamber 26. The electrochromic medium 24 is contained in space 26. The electrochromic medium 24 is in direct contact with transparent electrode layers 14 and 18, through which passes electromagnetic radiation whose intensity is reversibly modulated in the device by a variable voltage or potential applied to electrode layers 14 and 18 through clip contacts and an electronic circuit.
The electrochromic medium 24 placed in space 26 may include surface-confined, electrodeposition-type or solution-phase-type electrochromic materials. In an all solution-phase medium, the electrochemical properties of the solvent, optional inert electrolyte, anodic materials, cathodic materials, and any other components that might be present in the solution can be selected, such that no significant electrochemical or other changes occur at a potential difference which oxidizes anodic material and reduces the cathodic material other than the electrochemical oxidation of the anodic material, electrochemical reduction of the cathodic material, and the self-erasing reaction between the oxidized form of the anodic material and the reduced form of the cathodic material.
In most cases, when there is no electrical potential difference between transparent conductors 14 and 18, the electrochromic medium 24 in space 26 is essentially colorless or nearly colorless, and incoming light (fo) enters through front element 12, passes through transparent coating 14, electrochromic containing chamber 26, transparent coating 18, rear element 16, and reflects off layer 20a and travels back through the device and out front element 12. Typically, the magnitude of the reflected image (fR) with no electrical potential difference is about forty-five percent (45%) to about eighty-five percent (85%) of the incident light intensity (fo). The exact value depends on many variables outlined below, such as, for example, the residual reflection (f′R) from the front face of the front element, as well as secondary reflections from the interfaces between: the front element 12 and the front transparent electrode 14, the front transparent electrode 14 and the electrochromic medium 24, the electrochromic medium 24 and the second transparent electrode 18, and the second transparent electrode 18 and the rear element 16. These reflections are well known in the art and are due to the difference in refractive indices between one material and another as the light crosses the interface between the two. If the front element and the back element are not substantially parallel, then the residual reflectance (f′R) or other secondary reflections will not superimpose with the reflected image (fR) from mirror surface 20a, and a double image will appear (where an observer would see what appears to be double (or triple) the number of objects actually present in the reflected image).
There are minimum requirements for the magnitude of the reflected image depending on whether the electrochromic mirrors are placed on the inside or the outside of the vehicle. For example, according to some requirements from most automobile manufacturers, inside mirrors have a high end reflectivity greater than fifty-five percent (55%) and in some cases approximately of at least seventy percent (70%), and outside mirrors have a high end reflectivity of at least thirty-five percent (35%).
Electrode layers 14 and 18 are connected to electronic circuitry which is effective to electrically energize the electrochromic medium, such that when a potential is applied across the transparent conductors 14 and 18, electrochromic medium in space 26 darkens, such that incident light (fo) is attenuated as the light passes toward the reflector 20a and as it passes back through after being reflected. By adjusting the potential difference between the transparent electrodes, such a device can function as a “gray-scale” device, with continuously variable transmittance over a wide range. For solution-phase electrochromic systems, when the potential between the electrodes is removed or returned to zero, the device spontaneously returns to the same, zero-potential, equilibrium color and transmittance as the device had before the potential was applied. Other electrochromic materials are available for making electrochromic devices. For example, the electrochromic medium may include electrochromic materials that are solid metal oxides, redox active polymers, and hybrid combinations of solution-phase and solid metal oxides or redox active polymers; however, the above-described solution-phase design is typical of most of the electrochromic devices presently in use.
Others have the reflector on the third surface of the structure which simultaneously acts as an electrode for the electrochromic system. Over time, the reflective surface has changed from being on the fourth surface to being on the third surface. Silver or silver alloys such as silver gold are commonly used as the third surface reflector electrode. The thickness of the silver gold layer is commonly adjusted such that the system has a so-call transflective property, wherein the system has both appreciable transmittance and reflectance. Silver based materials are particularly well suited for this application due to their low absorption of visible light. Displays are often placed behind transflective systems. The transflective nature of the system shields the display from the viewer providing a stealthy characteristic.